Atrios and Zeos face oblivion in this disappointing season finale, saved mostly by a magnificent villain – the Shadow

"Astra, remember, you're the sixth princess of the Sixth Royal House of the Sixth Dynasty... And we're looking for the sixth segment of the Key to Time. Oh, you're in greater danger even than we imagined" - Romana

Storyline The Doctor, Romana and K•9 arrive on Atrios during the final stages of its war against twin planet Zeos. Armageddon is being orchestrated by a third party, the Shadow, who has the Atrian Marshal, Princess Astra and battle computer Mentalis all in his power. But the Shadow is also an agent of the Black Guardian. He knows the whereabouts of the sixth and final segment of the Key to Time and has been waiting for the Doctor to arrive with the other five…

RT Review by Patrick Mulkern So we've been following the Time Lords' quest for 20 weeks now, the Key to Time season arc is drawing to a close and we're all geared up for a satisfying denouement with the Black Guardian in a finale excitingly entitled The Armageddon Factor. What could possibly go wrong? Well, quite a lot actually. This is late 70s Doctor Who, remember.

Planning-meeting minutes (imagined): um, budget almost exhausted. So minuscule cast, please. No big names. No location filming. Not much dosh for effects, either. Planets under nuclear bombardment? Let's use distant crumping sound effects. Spaceship battles? Simple: flashes on a screen. This will have to be Armageddon on the scale of Space Invaders (a rudimentary 70s video game). And, umm, where was this Key to Time saga heading anyway? Oh well, let's prepare ourselves for another six weeks of sapping entertainment…

Too mean? Well, even the Doctor, in fruitless-runaround part five, sums events up: "Very crude, technically. Like all this amusement arcade rubbish." And there's no denying that Doctor Who's last six-parter hugely disappoints, yet it's not an unmitigated disaster.

The meandering plot fails to grip but does feed tiny surprises just as boredom sets in: the Marshal is communicating with a skull through his mottled mirror… "There are no Zeons on Zeos"… The opposition is run by a computer… The Shadow is so formidable even K•9 calls him Master… A rock wall opens and a comedy Time Lord pops out… Astra's preordained "transcendence"…

Director Michael Hayes achieves the best he can from limited resources. Decent camera moves maximise basic, gloomy sets (sometimes they double up or call for characters to run the long way around pillars). He uses CSO judiciously and numerous clever optical effects. Tom Baker and Mary Tamm enjoyed working with Hayes and work their socks off to enliven each situation - no matter how static.

The guest actors sadly have little to chew on. John Woodvine was asked to channel Churchill as the one-dimensional Marshal. Playing buffoonish Shapp, Davyd Harries seems to be asking himself: how would Bernard Cribbins approach this wretched scene? And poor Ian Saynor is saddled with Merak, for ten minutes an impassioned surgeon then for hours a lovesick drip bleating for Astra.

Lalla Ward, the daughter of a viscount, is well cast as the imperious princess, and is peculiarly adept at playing passion with minimal conviction. Of course, it's impossible to watch Ward now without the knowledge she'd become the next companion (and, for a blip, Mrs Baker); and it's disconcerting to see the two Romanas side by side on screen.

The Shadow lifts the story for me. Yes, his name and guise would befit a second-division Marvel Comics villain, but William Squire's vocal delivery is magnificently eerie. He gets the best dialogue by far, denigrating the Doctor's "jackdaw meanderings", freely revealing his allegiance to the Black Guardian - "He who walks in darkness."

Their aim for the Key to Time is to set "the two halves of the entire cosmos at war, and their mutual destruction will be music in our ears. Unlike others, it is not power we seek, but destruction that we glory in." We learn little else about the Shadow, nor his creepy henchmen, the Mutes. It's still unclear whether the name refers to their disfigurement, lack of speech or both.

The pervasive gloom and agonising time-loop footage of the third act are fortunately leavened by Drax, the cheeky-chappy Time Lord played so winningly by Barry Jackson. A sorta sarf London wheeler dealer, he and the Doctor met in "the class of 92", a tech course some 450 years earlier.

Amazingly, Drax at long last utters the Doctor's name, or at least his academy designation: Theta Sigma. If, in 1973, we accepted the Greek name Omega for another Baker/Martin Gallifreyan, surely we should also buy Theta Sigma or "Theet" for the Doctor…

Along with Garron, Professor Rumford and Grendel, Drax ("Remember me to Gallifrey!") is a vivid creation from season 16 I'd have gladly seen more of. He's arguably the last bright idea from writing duo Bob Baker and Dave Martin, who peppered 1970s Who with Axos, the Solonian lifecycle, Omega's black hole, crystalline Eldrad, K•9 … (Note: they give their robot dog plenty of action here.)

A shoestring Armageddon was never going to impress, and sources differ over whom to credit for making the sixth segment Astra. But the final somewhat garbled sequence - in which the Doctor goes eye-rollingly berserk, thwarts the Black Guardian and disperses the hard-won segments - was penned by incoming script editor Douglas Adams. It leaves the audience, who have invested 26 weeks in the Key to Time, gasping for answers.

In Adams' hands, will the universe - or indeed Doctor Who - be plunged into eternal chaos?